Radio messaging systems have utilized simulcast transmissions from multiple transmitters for providing radio coverage to large geographic areas. During a simulcast transmission a receiver positioned midway between two transmitters often can receive signals from both transmitters. The resultant instantaneous sum of the two signals depends upon their relative phase, and can be either larger or smaller than either signal alone. For example, if the two signals are substantially equal in amplitude and phase at the receiver, their resultant sum will be about twice the amplitude of either signal alone. If, however, the two signals are substantially equal in amplitude and 180 degrees out of phase, their resultant sum can be so small as to be undetectable by the receiver, due to destructive cancellation of the two signals.
Modern messaging systems utilize forward error correcting codes and bit interleaving to allow messages to be transmitted successfully in the presence of brief fades and noise bursts. Thus, an error-free message can be received even in the presence of intervals of destructive cancellation, provided that the intervals of destructive cancellation are sufficiently brief. To ensure that the intervals of destructive cancellation are sufficiently brief, the prior art messaging systems have employed a technique of permanently offsetting the carrier frequencies of adjacent transmitters with respect to one another by a small, fixed amount, e.g., 15 to 100 Hz.
A problem with the technique of permanently offsetting the carrier frequencies of adjacent transmitters is that it requires additional system planning and effort in setting up the radio messaging system. Furthermore, the technique can cause difficulties when adding new transmitters to an existing system, because the frequency offsets of many of the existing transmitters may have to be readjusted. In addition, some specific frequency offsets between adjacent transmitters, e.g., 200 Hz, are known to cause a higher word error rate, and should be avoided.
Thus, what is needed is a method and apparatus for limiting the intervals of destructive cancellation during simulcast transmissions. The method and apparatus preferably will limit the intervals of destructive cancellation without utilizing the prior art technique of permanently offsetting the carrier frequencies of adjacent transmitters with respect to one another.